1. Field of the Invention
The present invention relates generally to the field of wireless communications, and more particularly, to a method and apparatus for antenna impedance matching in a wireless communication device.
2. Description of the Related Art
It is well known that impedance matching is required to optimize energy transfer from a source to a load in radio-frequency circuits. In radio communication devices such as wireless transceivers, there are several places within the system requiring impedance matching, but one of the most challenging is the connection between a transmitter as a source and an antenna as a load. A poor impedance match in this part of a transceiver system results in the inefficient transfer of power to the antenna, and thus requires more power, for example from a battery in a portable system, in order to achieve a given level of radiated power needed for a robust communication link. A similar situation occurs between an antenna as a source and a receiver as a load, but mismatch in this path, while resulting in poorer received signal quality, does not have as direct an impact on power consumption of the transceiver. A parameter commonly specified to indicate the degree of mismatch is the voltage standing wave ratio (VSWR). An ideal VSWR of 1.0:1 indicates no mismatch (perfect matching) and no reflected power from a load.
Antenna matching is becoming more challenging in modern mobile wireless devices, which are supporting a growing range of services and frequencies beyond cellular telephony, including wireless local area networks (WLAN), personal area networks such as Bluetooth, mobile television protocols, and Global Positioning Systems (GPS). Miniature antennas are being asked to cover frequencies ranging from 824 to 2170 MHz and more in order to perform all these functions. Simultaneously, smaller batteries and longer battery life are desired in increasingly miniaturized, handheld packages, in which the antenna must be fit into available space and close to the outside of the housing, where it is susceptible to environmental effects. In mobile and handheld devices, the effective impedance of the antenna is not a constant, but continually changes as an operator walks, drives, changes hand position, or holds the antenna against his or her head or body. The impedance changes as power radiated from the antenna is reflected back by objects in the near vicinity. Although most RF components are designed to operate at a SWR of 1.0:1, in modern designs, an antenna VSWR of between 2.0:1 and 3.0:1 is usually specified as the compromise design match allowed by the constraints of available antennas and the various conflicting requirements. During operation of a handheld device, VSWR may degrade to as high as 9.0:1 without dynamic tuning of the impedance matching conditions, resulting in significant signal loss and power inefficiency. Thus a fixed impedance matching network is unable to maintain an optimal match over various orientations of the phone and environments around the phone, and methods are being developed to implement adaptive tunable matching networks with closed-loop control to dynamically adjust the tuning. Often different frequencies are used for transmit and receive functions, further complicating the impedance matching task by demanding either fast switching between transmit and receive configurations, by requiring simultaneous optimization for both transmit and receive using duplicate components, or by accepting a compromise between transmit and receive. Independent tunable matching networks could be implemented for the transmit and receive functions, but this would incur twice the cost. This is undesirable, because in addition to improvements in performance and miniaturization, there is also pressure for designs to achieve continual cost reductions.
A number of solutions for dynamic impedance matching in wireless transceivers have been suggested. Handset designers have experimented with microelectromechanical systems (MEMS) technology to implement RF switches or tunable capacitors. Nonlinear capacitors using nonlinear ferroelectric dielectric materials such as barium strontium titanate (BST) can be tuned using a bias voltage. Both of these tunable components require high bias voltages to tune, up to 30 V or higher, requiring additional DC-to-DC converters to multiply the battery voltage, and cannot be easily integrated with RF, analog, and digital circuitry on the same die. Digitally-tunable capacitors (DTCs) have also been proposed for integration on silicon on sapphire substrates. These various tunable components can be incorporated into tunable antennas and filters, but they require high RF power handling capability in the transmit path after the power amplifier and leading to the antenna. Moreover, in themselves they do not provide the sensing mechanism to detect the VSWR that is needed to determine the desired settings for the tunable devices. Thus another component is needed, such as a directional coupler that can pick off both forward and reflected signals from a transmission line. The signal from the directional coupler is interpreted by an electronic circuit to close the tuning loop. The tuning controls are adjusted electronically though a control loop such that the reflected power is reduced to a minimum while the forward power is maximized. These tuning and control circuits are either designed in the same substrate/process as the power amplifier or on one or more independent ICs. Directional coupler structures take up precious space, either as separate components, or as incorporated into the transmission lines on a printed circuit board.
Examples of existing solutions for the detection and tuning of antenna mismatch are described in U.S. Pat. No. 6,845,126 to P. W. Dent and R. A. Dolman, issued Jan. 18, 2005 (hereinafter, “the '126 patent”), U.S. Pat. No. 6,961,368 also to P. W. Dent et al., issued Nov. 1, 2005 (hereinafter, “the '368 patent”), and an article by R. Novak and T. Ranta, “Antenna Tuning Approach Aids Cellular Handsets,” in Microwaves & RF magazine, November 2008 (available online at http://www.mwrf.com/Index.cfm?ArticleID=20085). In the '126 patent, a directional coupler is used to direct reflected signals to a homodyne receiver that down-converts the reflected signals to baseband to detect the antenna mismatch during a transmit mode and down-converts the received signals during a receive mode. The down-converted reflected signals are used in a baseband processor to generate control signals for a programmable matching network. In the '368 patent, a directional coupler is also employed to pick off a reflected signal for an impedance mismatch measuring and quantizing unit to use in configuring an adjustable matching network and various switches. Novak and Ranta describe digitally tunable capacitors integrated using silicon on sapphire substrates as an alternative to MEMS and BST capacitors, and also assume the use of a directional coupler to pick off from the antenna forward and reflected signals whose powers are then detected. These existing solutions all require additional complexity, and therefore cost, in the form of additional separately-packaged components to separate and sense the forward and reflected signals from the antenna, as well as to perform an adjustment of the impedance feeding the antenna.
There is accordingly a need to further improve the detection of antenna impedance mismatch in wireless transceivers, as well as to implement new solutions for adjusting the impedance, thereby improving power consumption and thus battery life, especially in handheld transceivers. Tunable impedance also simplifies the design of the power amplifier and its housing to dissipate heat, as the total radiated power requirement can be met by antenna tuning rather than by using the brute-force approach of increasing the output power to make up for the loss of radiated power due to reflected power. It is particularly desirable to find solutions that can be integrated into multifunction integrated circuits, thereby reducing size, eliminating components and thus lowering the cost.